The present invention relates to an electrolytic cell and in particular to an electrolytic cell of the type used for the oxidation of nickel (II) hydroxide, having a container for the electrolyte, as well as anodes and cathodes fitted at a short distance from each other overlappingly and connected to a source of power by means of lugs.
The oxidation of nickel (II) hydroxide to nickel (III) hydroxide requires a considerably high oxidation potential, but it can be performed using certain chemicals such as per-sulfate, chlorine an ozone, or electrolytically in an oxidization cell suited for this purpose.
Often the oxidized product, nickel (III) hydroxide, is further used for the oxidation and precipitation of impurities such as cobalt, iron, manganese, lead, arsenic, selenium and bismuth from electrolytic solutions, for example a nickel electrolyte.
The oxidizing capacity of nickel (III) hydroxide is, of course, better when the degree of oxidation from the initial product is higher. In practice it has been observed that the oxidation can be carried out beyond the stage of nickel (III) hydroxide, in which case the product also includes nickel (IV) compounds. The oxidizing capacity of such a product is especially high.
The oxidation of nickel (II) hydroxide to nickel (III) hydroxide by means of chemicals is not always advantageous. A few reasons for this are given below:
Effective chemicals are expensive and their use usually requires a stoichiometric excess if the aim is a product in which the oxidation has been carried out at least to a degree of oxidation corresponding to nickel (III) hydroxide.
The use of chemicals may be detrimental to other operations within the process; they can, for example, accumulate in the process or corrode the apparatus.
The degree of oxidation achieved in a nickel hydroxide precipitate by using inexpensive chemicals (e.g. a gas mixture of oxygen and sulfur dioxide) is usually low, and in this case its oxidizing capacity is not high.
The electrolytic oxidation of nickel (II) hydroxide can be performed without adding any detrimental chemical to the process. When using the prior known techniques, however, the current efficiency of the electric energy used has been only 15-20% when the oxidation has been carried out to the nickel (III) hydroxide. The present invention relates to a new electrolytic cell by means of which nickel (III) hydroxide can be prepared in such a manner that the efficiency of the current used is many times higher than the previous one.
In an electrolytic oxidation process the hydroxide particle being oxidized behaves in accordance with Reaction (1): EQU Ni(OH).sub.2 +H.sub.2 O.revreaction.Ni(OH).sub.3 +H.sup.+ +e.sup.-( 1)
Since a Ni(OH).sub.2 particle is electrically neutral externally, it does not behave in the electrolyte in the same way as ions. The particle to be oxidized is brought to the anode surface by mixing the solution/solid suspension by a compressed-air blast, for example.
An anode reaction (2) EQU 2H.sub.2 O.revreaction.O.sub.2 (g)+4H.sup.+ +4e.sup.- ( 2)
useless in terms of the final result competes with reaction (1).
In this case, whether or not oxidizing particles can be brought to the anode surface in the quantity required by the consumption of power is crucial for Reaction (1). In other cases Reaction (2) occurs as the anode reaction.
Considering the final result, it is therefore important that the particles being oxidized have a high chance of meeting the anode surface. This chance is created when in the oxidation cell there is a maximal anodic surface area in proportion to the hydroxide particles present in the suspension and when the mixing is advantageous in terms of the movement of the particles.
In previously known electrolytic cells the electrodes have been suspended from electrode arms, along which electricity is also conducted to the electrodes. In practice such a technique limits the placing of the electrodes close to each other without producing short-circuits during the electrolysis owing to the electrode arms and the bolt attachment of the electrodes extending through the arms. By careful use of this technique only 25-30 m.sup.2 of anodic surface area is obtained per one cubic meter of the hydroxide suspension to be oxidized.
The object of the present invention is therefore to provide an electrolytic cell in which the electrode surface area/tank volume ratio is higher than previously and thereby the current efficiency of the electrolytic cell is higher than previously.